Related Art Statements
This invention relates to a method for reproducing data recorded on an optical card.
In an optical card, data is recorded as arrays of pits whose pitches are modulated in accordance with the data. In case of reproducing the data thus recorded on the optical card, the optical card and a reading head are moved reciprocally relative to each other to derive output signals both in a forward movement and in a backward movement. In a known reproducing method described in U.S. Pat. No. 4,525,822 issued on June 25, 1985, distances between successive pits are detected by using front edges or rear edges of pits as a positional standard regardless of the direction of reciprocal movement.
In case of recording the data on the optical card, a shape of a pit recorded by a laser beam becomes asymmetrical at front and rear edges viewed in a write-in direction due to the property of record medium of the optical card. FIG. 1 shows a typical configuration of pits recorded on the optical card. In this example, rear edges of pits viewed in the write-in direction are widened. It should be noted that the rear edge means an edge which is recorded at a later timing. In FIG. 1, r.sub.1, r.sub.2, r.sub.3 and r.sub.4 denote distances between front edges of successive pits P.sub.1, P.sub.2, P.sub.3, P.sub.4 and P.sub.5, respectively, and t.sub.1, t.sub.2, t.sub.3, t.sub.4 represent distances between rear edges of the pits P.sub.1, P.sub.2, P.sub.3, P.sub.4 and P.sub.5, respectively. As explained above, when the rear edges fluctuate, the distances t.sub.1 .about.t.sub.4 include more jitter than r.sub.1 .about.r.sub.4.
In the known reproducing method, the front or rear edges of pits are detected regardless of the write-in direction. For instance, when front edges of pits viewed in the read-out direction are detected, in a forward movement in which a read-out direction is the same as the write-in direction, the distances r.sub.1 .about.r.sub.4 are detected so that the reproduced signal contains only a small jitter. However, in a backward movement in which the read-out direction is opposite to the write-in direction, the distances t.sub.1 .about.t.sub.4 are detected and thus relatively large jitter is introduced in the reproduced signal. Therefore, in the known reproducing method, the reproduced signal contains large jitter and the error rate becomes worse.